The preparation of cast cheese has been known since 1969, cf. for instance U.S. Pat. No. 4,205,090 (Maubois et al.). In principle, this process is characterized by separating the milk product in to a filtrate (permeate) and a liquid retentate by membrane filtration, such as ultrafiltration (UF). Apart from casein, the retentate also retains water-soluble proteins, i.e. the so-called whey proteins comprising globulin and albumin, while lactose and non-protein nitrogen compounds (NPN) remain in the permeate. The retentate has substantially the same dry matter content as the finished cheese. To make cheese the retentate is processed by adding cheese rennet and starter, whereupon the resulting product is cast into cheese molds, brine salted and cured.
Examples of developments of the above process of preparing cast cheese include the technique disclosed in GB patent application no. 2.105.167 (Qvist et al.), and SE published accepted application no. 390.787 (Maubois et al.). None of the processes disclosed in the above specifications uses a filtration retentate having a dry matter concentration in the range of approx. 38% by weight or above. The retentates according to the examples of the above specifications have a dry matter content of approx. 30% by weight and 20% by weight, respectively.
GB patent specification no. 2.101.866 (Rubin et al.) discloses a process for preparing a so-called cheese-base by ultrafiltration combined with diafiltration (DF) of milk followed by evaporation. The retentate is inoculated with a starter prior to and subsequent to evaporation. The process is used to prepare a raw material suitable for the preparation of processed cheese. However, the process is unsuitable for the preparation of classical,semi-hard cheese.
The known process according to the above U.S. Pat. No. 4,205,090 (Maubois et al.) is widely used for the preparation of cheese having a comparatively low dry matter content. The process has, for instance, been commercially successful when preparing feta-cheese. The process has, however, been useless for the preparation of so-called semi-hard yellow cheese. By semi-hard yellow cheese is meant a semi-hard cheese having a dry matter content of at least 38% by weight in the case of a 20+ cheese (i.e. a cheese where the fat makes up 20% by weight of the total dry matter content) or at least 41.3, 46.3 or 48% by weight, respectively in the case of 30+, 45+ or 50+ cheese. A reason why the known method has not previously been usable for the preparation of a semi-hard yellow cheese is that a retentate of membrane filtration having a higher dry matter content becomes very viscous when cooled to temperatures below 40.degree. C. As a result, the subsequent addition of starter and rennet is difficult to carry out satisfactorily. The admixing is often insufficient resulting in unacceptable products, as coagulation is not uniform. As a result, areas of excess coagulation are formed and thereby a risk that so-called onion rings are formed. This means that the cheese does not cohere and tends to split. Furthermore, this results in undesired whey separation, thus wasting valuable proteins in the whey. The high viscosity further results in an increased load on the plant used and causes great pressure losses.
It is known that to avoid unwanted growth of microorganisms it is desirable to carry out a heat treatment as late as possible. It would thus be most desirable to carry out heat treatment immediately prior to adding starter and rennet. However, when subjecting the retentate to heat treatment, this may result in a further increase in viscosity and thus enhance the above problems associated with increased viscosity. Furthermore, persons skilled in the art have generally been of the opinion that such heat treatment of the retentate might have a destructive effect on the quality of the cheese and result in a bitter cheese.
Furthermore, a large number of articles have been published relating to the preparation of cheese by membrane filtration. R. de Boer and P.F.C. Nooy of Niederlandisches Institut fur Milchwirtschaftliche Forschungen, Ede, Netherlands: "Low-fat semi-hard chees from ultrafiltered milk", Nordeurop isk Mejeritidsskrift, no. 3/1980, discloses the preparation of semi-hard cheese by ultrafiltration. In the article the two authors mention the viscosity of skimmed milk concentrates of high protein content. "Studies on the viscosity of such concentrates having a protein content of up to about 20% point to a pseudoplastic behaviour. This pseudoplastic behaviour is largely influenced by the rate of shear. . . " (the paragraph bridging page 58 and page 59). The article discusses the problems connected with viscosity during the preparation of cheese by ultrafiltration of skimmed milk followed by evaporation. It is mentioned that for a protein content of 31% a proper control of the temperature in the scraped-surface evaporator is necessary to avoid gelation. When cooling to renneting temperature the viscosity increases. It has been found that at certain rates of shear the influence of the decrease in temperature on the viscosity is only limited. Thus a satisfactory mixing of rennet and starter concentrate can be carried out by sufficiently high rate of shear. This process is complicated, as highly concentrated cream is added later and two thus viscous production lines are thus involved, one with skimmed milk and one with highly concentrated cream. This among other things results in a more complicated control of the process. Furthermore, when working with the highly concentrated cream, i.e. cream with 70-80% by weight of fat there is a risk of the phases of the emulsion inverting (from oil-in-water to water-in-oil), whereby butter is produced.
U.S. Pat No. 4,355,048 (Schaap et al.) discloses the preparation of semi-hard cheese, such as Gouda and Edam, by ultrafiltering skimmed milk to obtain a retentate having a dry matter content of at least 31.5%, such as in the range of from 31.7 to 32.4%. The retentate is then mixed with fat, such a milk fat, and rennet and starter are added, whereupon the resulting material is cast, pressed, salted and cured in a conventional manner. Prior to casting, a dry matter content of 38% by weight for a 20+ cheese and of 48% by weight for a 50+ cheese is obtained due to the addition of fat. In the disclosed process, the high dry matter contents are thus obtained by subsequent addition of fat and, a retentate having the final dry matter content is thus not prepared by ultrafiltration. Also here, two highly viscous production lines are involved. Furthermore, no heat treatment of the retentate is carried out.
The article by R. Delbeke of Government Dairy Research Station, Melle, Belgium: "Experiments on making Saint-Paulin by full concentration of milk with ultrafiltration", Milchwissenschaft 42 (4) 1987, p. 222-225, discloses experiments relating to the preparation of Saint-Paulin cheese by ultrafiltration of milk to obtain a retentate having an average dry matter content of 46%, whereof 21% are protein and 21% fat. The retentate resulted in a cheese having a dry matter content of 50% after 4 weeks without whey drainage. In particular, the article points to the influence of various parameters of the process on the final product, especially calcium content, pH-value, buffer capacity, lactose and ash content, pre-acidification and sodium chloride treatment. At the relatively low dry matter content (46%) for a fat content of about 45% of the dry matter combined with an addition of 0.5% sodium chloride, this method conventionally does not result in problems of viscosity. It is of course also a contributory factor that the retentate is not subjected to heat treatment. The process requires acidification in a tank and can therfor hardly be carried out continuously.
The article by H. Gouderanche, J. L. Maubois et al.: "Utilization of the new mineral UF-membranes for making semi-hard cheeses", Desalination, 35 (1980), pp. 234-258, discloses the preparation of semi-hard cheese, such as Saint-Paulin, having a protein content of 21% and a dry matter content of 45% by membrane filtration. The process is based on the use of an inorganic UF-membrane. Fat-standardized milk is ultrafiltered and diafiltered to a protein content of approx. 21% and a dry matter content of approx. 45%. The retentate is then cooled to 30.degree. C. and either conventional or freeze-dried starter is added. At a pH-value of approx 5.2 first 1.0% sodium chloride and then rennet is added in an amount of approx. 40 ml each 100 kg retentate. In this process the problems associated with the viscosity may more easily be overcome because of the pH-decrease and the addition of salt. However, the above process requires a period in a tank in order to ensure the process of acidification. The above is thus a rather complicated process which can hardly be carried out continuously.
P. Ducruet, J.-L. Maubois, H Goudedranche and R. Pannetier, "Elements de fabrication de fromages a pate demi-dure et a ouverture propionique selon le procede MMV", La Technique Laitiere No. 957, September 1981, pages 13-16 discloses a further development of the above technique according to Goudedranche et al., where the addition of salt is, however, not used. The dry matter content of the retentate is 45-47% by weight. No heat treatment of the retentate is carried out and to ensure a sufficiently low viscosity, the renneting is carried out at 40.degree.-45.degree. C., which requires the employment of particular thermophilic starter cultures. Furthermore, Ducruet uses pre-acidification, the starters being added 30 minutes before the rennet. This makes it difficult to carry out the process continuously.
In order to avoid the drawbacks of the highly concentrated retentates subjected to heat treatment, it has been attempted to admix starter and rennet at elevated temperatures, such as 42.degree. C., thus decreasing the viscosity. Such elevated temperatures, however, prevent the use of conventional starter cultures, for which reason one has to resort to special, thermophilic cultures. These special cultures usually result in an atypical taste and structure of the finished cheese, which is not desirable in some types of ripened cheese.
It has also been attempted to increase the amount of starter and rennet mixture by adding more water. This measure was equally unsuitable, since it diluted the product. If it is thus desired to obtain a finished cheese with a predetermined dry matter content it is necessary to carry out membrane filtration to an increased dry matter content. Consequently the membrane filtration capacity has to be increased, and the obtained retentate has an increased viscosity as a result of the increased dry matter content.
It has surprisingly been found that; it is possible to prepare cast cheese having a dry matter content corresponding to the one of conventional semi-hard cheese, i.e. cheese prepared by firstly coagulating the milk, followed by separating the whey by use of conventional rennet and conventional starters without the above problems, thereby obtaining a cheese of good quality, taste and consistency. To this should be added that it has even been found to be possible to subject the retentate from the membrane filtration to heat treatment immediately prior to the admixture (dosing) of rennet and starter, without encountering problems of increased viscosity.